Hereditary angioedema is a disease inherited as an autosomal dominant trait. It is characterized by abnormality in a serum protein, the C1 esterase inhibitor. We show that danazol corrected the protein abnormality in patients with this disease. During this year we have studied the toxicity of danazol and the effectiveness of the drug in a large patient population. We have determined the minimally effective dose and have defined a whole new series of toxic side effects of danazol. The drug is still considered relatively non-toxic in this patient population at high risk of death from their disease. Acute attacks of hereditary angioedema which do not respond to danazol have been treated with the intravenous infusion of the purified C1 esterase inhibitive protein and this has stopped the attack within hours of the time of the infusion.